This invention relates to the controlled use of supplementary fuels in compression ignition engines and more particularly to a system that includes a variable area venturi in the main air supply to the engine for creating a variable downstream vacuum which variable draws a constant porportion fumigation fuel/air mixture into the main airstream to the engine in response to variations in engine load and speed.
The supplemental fueling of a diesel engine by feeding fuel vapors into the main intake air (herein referred to as fumigation) is known to improve power output and service life while reducing fuel consumption, pollution output and noise level of the engine. Fumigation also permits lesser quality primary fuels to be used to fuel a diesel engine in substitution for at least a portion of the higher quality diesel fuel normally required. Although the fumigation fuel may be the same as the primary engine fuel, in most applications the fumigation fuel will be a different fuel such as propane.
Various fumigation systems have been implemented to achieve these results. For example U.S. Pat. No. 3,698,365 to Hallberg discloses one fumigation system widely practiced on a variety of farm tractors. In Hallberg, a fumigation fuel, such as propane, is metered through a small spud, located in the air intake, to the air inlet manifold of a diesel engine. The metering device is a rotary barrel type of valve having a V-shaped slot orifice in the barrel at right angles to the axis of the barrel. As the barrel is rotated, the V-shaped slot progressively opens or closes to the passage of propane through the valve. Propane is supplied to the valve from a propane source at high pressure. The pressure is reduced to a useable level by a conventional pressure regulator. The valve is mechanically linked to the diesel injection control which controls the speed of operation of the diesel engine. The spud is located and arranged so as to achieve a suitable mixture distribution of the supplemental propane fuel vapor to each cylinder.
Various applications and modifications of the fumigating technique disclosed in Hallberg have been designed and built into the control valve to facilitate linkage simplification. However, devices to accurately and reproducibly deliver fuel according to a predefined schedule with adequate fuel distribution in multi-cylinder engines has proved to be difficult to achieve with simple spud arrangements. Compromises to alleviate such difficulties have resulted in suboptimal operation. Additionally, installation of Hallberg type systems is difficult and mechanical reliability frequently unacceptable.
Another technique for diesel engine fumigation disclosed by Govindara et. al. in SAE Paper No. 810347 utilizes two valves coupled to a Y or T connector on the air intake manifold of the engine. The first valve controls the flow of air from the atmosphere. The second valve is part of a vapor carburetor that automatically prepares a uniform fumigation fuel/air mixture. The two valves are linked, generally in a non-linear manner, to the diesel injection control and to each other. Compromises to the ideal fumigation fuel schedule are generally made as a means of avoiding mechanical complications in the multiple linkages. Such compromises result in limitations in fumigation and hence engine operation.
Diesel engine fumigation has also been used in diesel driven irrigation pumps where the engine operates at a constant speed. One such system is the McClure-Deutz System described in an article entitled "Pilot Fuel Improves Irrigation Economies" by Bill Roberts, Diesel Progress North America, March, 1983, at pages 53-55. Such irrigation pump systems avoid the mechanical complexity of either of the two prior methods by carbureting the entire engine air flow with the fumigation fuel. However, such systems have unacceptable characteristics for engines operating at varying loads and speeds as is the case with motor vehicles.
By contrast to the above systems, the present invention comprises a venturi with a throat which has a variable opening which allows for fumigation over a wide range of operating loads and speeds with the proportion of fumigation fuel drawn into the engine being controlled by the engine air flow or suction. This air flow causes variations in the proportion of fumigation fuel in the air drawn into the engine according to a preset schedule over the range of engine loads and speeds. The resultant system is simple and easily retrofitted to existing engines. The variable opening venturi is also adjustable to enable the attainment of various schedules of fumigation fueling depending on operational desires and engine ratings.